The invention relates to a method for operating an electrically controllable hydraulic valve, in particular an analog/digital inlet valve of a wheel brake in a motor vehicle brake system, wherein an actuating current is applied to the hydraulic valve.
Brake systems of motor vehicles typically comprise a number of friction brakes being associated with the wheels of the motor vehicle. The brake pressure required for braking is usually built up by way of a brake pedal using a so-called booster, and is conducted by way of a master brake cylinder by means of brake fluid in hydraulic lines to the friction brakes or the brake cylinders, respectively. Brake pressure can also be built up by means of appropriate motor-and-pump assemblies in systems with brake force boosting. In conventional brake systems, the position of the brake pedal usually corresponds, however, with the position of the friction brakes or the braking performance of the motor vehicle, respectively.
In modern motor vehicle brake systems, it is however frequently necessary to electronically modify or electronically control the brake pressure, which prevails at the friction brakes, in order to realize safety systems, such as an anti-lock system (ABS) or an electronic stability program (ESP). Thus, a comparatively high-frequent brake pressure request of pulse-type variation prevails in an anti-lock system in order to prevent locking of the wheels and, thus, minimize the stopping distance. In this arrangement, friction brakes are released in the event of wheel lock by a reduction of the brake pressure and are re-applied shortly thereafter by an increase in pressure.
A dynamic control of the brake pressure of this type is typically realized by means of electrically controllable hydraulic valves, which are connected into the hydraulic lines that lead to the friction brakes. For regenerative brake systems equipped with an electric generator in addition to the friction brakes, valves of this type are also used in addition to other components in order to realize the brake functions. In so-called ‘brake-by-wire’ brake systems, wherein the friction brakes are maintained hydraulically uncoupled from the brake pedal, the detected braking request is realized or controlled also by way of electronically controllable hydraulic valves in the brake system.
The electronically controllable valves are usually actuated by way of an electronic control unit. In up-to-date generations of hydraulic control devices, so-called analog/digital valves are used at an increasing rate. These valves are customary switching valves, which can be operated in such a fashion though that analog pressure control can be performed. The valve is then moved by way of an electric coil through which alternating current is usually flowing. The valve movement induced by the magnetic field of the coil or the valve control, respectively, corresponds with the effective value of the coil current. Inlet valves of wheel brake cylinders are usually designed in such a way that a valve is open in its deenergized state so that the maximum opening cross-section of the valve is adjusted in the deenergized state. Actuation of such a valve by an actuating current causes the valve to continuously close with an increasing actuating current until the valve is closed completely when the so-called opening current is reached. In order to keep a valve of this type closed in a reliable manner, it is acted upon by an actuating current that is in excess of the opening current.
The opening current of a valve of this type usually depends on the difference in pressure across the valve. To be able to actuate a valve of this type e.g. in a brake control system in conformity with its targets and requirements, usually so-called opening current characteristic curves are stored in an associated control unit, in which the dependency of the opening current on the difference in pressure is represented in table form or also functionally.
In the operation of a valve of this type, it is typically acted upon by an actuating current, which should correspond with the desired opening position of the valve to a largest possible degree. Initially, the valve is acted upon by this actuating current in the way of a pilot control, and as the case may be, this current valve can be adapted by a subsequent control operation depending on determined actual parameters. Resorting to the opening current characteristic curve that is already provided, a valve of this type usually is initially acted upon by an actuating current, in consideration of the pressure gradient, which actuating current corresponds to the respective opening current, multiplied by an invariably selected correction term of e.g. 0.8. The control current, which is required for the adjustment of the actually desired opening position, is adjusted thereafter by way of the subsequent control.
However, it has turned out that especially for electronically controlled brake systems, e.g. in connection with ABS or ESP systems, the necessary recovery times or floating times can be comparatively long and, thus, can impair the operability of the mentioned system.
In view of the above, an object of the invention is to disclose a method of the above-mentioned type, which allows pressure control with a particularly quick reaction at a highest possible rate of accuracy using the valve described hereinabove. Another objective is to disclose a brake system that is well suited for implementing the method at issue.